This invention relates to a device such as an instrument for optically, preferably visually determining a definite plane.
Such an instrument is of the kind as set forth in the U.S. Pat. application Ser. No. 841,121 now U.S. Pat. No. 4,166,699.
The instrument according to said U.S. Patent is of such a nature, that by observation of the instrument information is obtained as to which side of a certain plane defined by the instrument the observing person or an object is located, or whether the observing person or the object is located in said plane. The instrument comprises screens of opaque lines, which are separated by transparent interspaces. The screens are disposed in such a manner relative to each other that a certain distinct interference pattern, a so-called moire pattern, is formed when the observer or an object is in said plane, and that other distinct moire patterns are formed when the observing person or an object is outside said plane. The screens usually are located in a box of sheet metal or another rain- and wind-proof material. In the rear end wall of the box a light opening is provided. In such a device light is transmitted through the screens to the observer who thereby sees the moire patterns which have been formed.
The instrument is used mainly outdoors, for example for levelling the ground at road and building construction sites, etc., in which cases the plane defined by the instrument preferably is parallel with the desired ground plane.
When the instrument is used outdoors, it is difficult during parts of the day and for long periods of the year to see the moire pattern, because the intensity of the light transmitted through the screens is too low. In order to increase the intensity, artificial lighting can be applied at the light opening on the rear side of the instrument. The artificial lighting, however, when it is used during large parts of the day as it must be during the winter term, causes extra troubles with respect to the necessary availability of lighting accessories, exchange of batteries, lamps etc.
It is, therefore, desired to reduce the time when artificial lighting is required.